The milk should taste considerably different because the small quantities of juice from the lemon have large quantities of Citric Acid. Citric Acid is common in many foods because it cheap and potent (meaning that it has a strong taste).

Acidity is a measurement of the activity of hydrogen atoms in a molecule. This is measured on something called a “pH” scale, where anything below a 7 is an acid, anything above a 7 is a base. When the acid and base mix, they create a reaction, which breaks bonds between the molecules of the acids and bases. In some cases, this reaction can produce heat (which is why many cartoons show acids as being able to burn through steel and other materials).